1. Technical Field
This invention relates a catalyst support material useful in an electrode assembly for an electrochemical fuel cell that also includes an ion exchange membrane and at least one (and most commonly two) electrode, sometimes referred to as a gas diffusion layer. The electrode assembly comprises a catalyst material, typically platinum or a platinum group metal, which is present on a catalyst support material that comprises elongate anisotropic particles of expanded graphite.
1. Background of the Invention
An ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen with oxygen in the air. Within the fuel cell, electrodes denoted as anode and cathode surround a polymer electrolyte. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
A PEM fuel cell is advantageously formed of a membrane electrode assembly sandwiched between two graphite flow field plates. Conventionally, the membrane electrode assembly consists of random-oriented carbon fiber paper electrodes (anode and cathode) with a thin layer of a catalyst like platinum or a platinum group metal coated on isotropic carbon particles, such as lamp black, bonded to either side of a proton exchange membrane disposed between the electrodes. In operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which joins with the protons through the proton exchange membrane and the electrons through the circuit, and combine to form water. Since the membrane is an insulator, the electrons travel through an external circuit in which the electricity is utilized, and join with protons at the cathode. The air stream on the cathode side removes the water formed by combination of the hydrogen and oxygen. Combinations of such fuel cells are used in a fuel cell stack to provide the desired voltage.
One possible limiting factor to the more widespread use of PEM fuel cells is the cost of the catalyst material and the relatively high resistivity of the isotropic carbon support material. In conventional fuel cells, the catalyst material is bonded to the PEM or the electrodes in a nonselective manner. That is, the catalyst material is distributed relatively uniformly on the opposed surfaces of the PEM, with result that catalyst deployment is not as efficient as possible.
Unlike the carbon materials conventionally used as a catalyst support material, graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphite exhibits anisotropy because of its inherent structure and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing expanded graphite possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Graphites such as Kish graphite and highly oriented pyrolytic graphite, but especially natural graphite, can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated flexible graphite sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is at least about 80 times the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material is believed to be possible due to the excellent mechanical interlocking, or cohesion which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. sheet, web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting expanded graphite under a predetermined load and in the absence of a binder. The expanded graphite particles, which generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.08 grams per cubic centimeter (g/cc) to about 2.0 g/cc.
The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
When electrically anisotropic flexible graphite sheet material is shredded and milled to provide elongate particles sized predominantly about 2 to 50 microns in length, the particles are anistropic with respect to electrical resistivity, with the resistivity along the length of the particles being very low compared with the resistivity transverse to its length, making the particles uniquely effective as a fluid cell catalyst support. In addition, the sheet-like nature of the elongate particles bring about better particle to particle connectivity, and hence better electrical conductivity, compared to more spherically shaped conventional carbons.
The present invention provides a catalyst support material useful in a membrane electrode assembly for an electrochemical fuel cell, the electrode assembly comprising a pair of electrodes and an ion exchange membrane having opposed surfaces positioned between the electrodes. The inventive catalyst support material is formed of elongate electrically anisotropic particles milled from flexible graphite sheet material, such as flexible graphite foil or mat.
In a preferred embodiment, at least one of the electrodes of the membrane electrode assembly is formed of a sheet of a compressed mass of expanded graphite particles having a plurality of transverse fluid channels passing through the sheet between first and second opposed surfaces of the sheet, one of the opposed surfaces of the sheet abutting an opposed surface of the ion exchange membrane. The transverse fluid channels are adjacently positioned and separated by walls of compressed expanded graphite. At least a portion of an opposed surface of the ion exchange membrane is in operative contact with a catalyst material that is supported on the inventive catalyst support formed of elongate electrically anisotropic particles milled from flexible graphite sheet material.